1. Field of the Invention
The present invention relates to a lithographic apparatus and a position quantity determination system and method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the state of the art it is known to determine a position of a movable part of the lithographic apparatus making use of an interferometer. The interferometer emits an optical measurement beam from a source to the movable part. The measurement beam is then, e.g., reflected by the movable part and detected by a detector. Also, the measurement beam, or a part thereof, may be directed from the source to the detector via another optical path. At the detector, the two beams arriving there will interfere resulting in a wavelength related readout. Upon movement of the movable part, a distance traveled by the measurement beam from the source via the movable part to the detector will change, thus resulting in a phase shift of the measurement beam when arriving at the detector, which results in a change in the interference pattern. Making use of the interferometer, accurate position measurements, whether absolute or relative, depending on the type and construction of interferometer used, may be performed. It is know that the readout of the interferometer is related to a wavelength of the measurement beam, which in turn depends on physical parameters of a fluid, such as a gas or a liquid, through which the measurement beam is transmitted. In other words, readout of the interferometer is dependent on a wavelength of the measurement beam. The wavelength of the measurement beam depends on physical parameters, such as a pressure or a temperature of the fluid via which it is transmitted. It is known to measure such physical parameter, making use of a sensor, e.g., a pressure sensor in case that the physical parameter includes a pressure or a temperature sensor, in case that the physical parameter includes a temperature.
The above solutions may work sufficiently well in a static environment where no movable parts are present or where an insignificant effect of any movements of movable parts on the fluid is observed. In a lithographic apparatus, however, structures may be present which may have comparably large dimensions and which may move at a comparably high speed and with a high repetition frequency. Examples of such movable structures (in other words movable parts) may include a substrate table, a measurement table in a dual scan configuration, or any other movable structure. The movable structure may be the same as the one of which a position is determined by the interferometer system, however it is also possible that the interferometer system is arranged to measure a position of one movable part, while another movable part of the lithographic apparatus may cause local (e.g. dynamic) variations of the physical quantity, and deteriorating an accuracy of the interferometer system.